The invention relates to a method of separating suspension, in particular for treatment of waste water, wherein the flocculating suspension is separated from the liquid by filtration in a fluidized layer in a sludge blanket wherein the flocks are created from the separated suspension and the fluidized state is maintained by the rising stream of liquid, while the liquid with suspension enters the fluidized layer from the bottom and the liquid freed from suspension is discharged above the surface of the sludge blanket represented by the interface between the fluidized layer and the liquid without suspension. Further it relates to an apparatus for performing this method containing an upward widening separator that is provided with the inlet of liquid with suspension in its bottom part, and a means for withdrawal of liquid without suspension in its upper part.
One of the most advanced methods for the separation of flocculating suspension during the purification and treatment of water is fluid filtration in a sludge blanket. The sludge blanket consists of a fluidized layer of flocks that are created by agglomeration of particles of the separated suspension. Water with suspension to be removed enters the sludge blanket by upward streaming. This flow sustains the layer of flocks in fluidized condition. During the throughflow of water with suspension through the fluidized layer the particles of suspension contact flocks with following capturing of suspension particles due to their adhesion to the flocks. This filtration relieves water from suspension that is transformed into flocks that are substantially larger than the inflowing suspension particles.
The fluidized layer creates a top interface between the fluidized layer and the liquid without suspension, the so-called surface of sludge blanket, the liquid freed from separated suspension being withdrawn above the surface of sludge blanket. The interface is established, if the flow velocity of liquid directly above the interface is lower than the velocity of not retarded sedimentation of separate particles creating the fluidized layer. Since the flocks created in the sludge blanket by agglomeration of suspension are substantially larger than the particles of inflowing suspension, this velocity substantially exceeds the sedimentation velocity of the separated suspension. The withdrawal of clear liquid shall be sufficiently spaced from the surface of sludge blanket, for preventing flocks from being dragged out of the sludge blanket due to irregularities of withdrawal. Due to that a layer of clear liquid in the separation zone above the sludge blanket is always indispensable.
The fluidized layer shall be supported from the bottom. A frequently used method of supporting the fluidized layer is hydrodynamic support consisting in that the quick flow of liquid under the fluid layer prevents its fall down. In such case the flow velocity of liquid in the fluidized layer decreases in upward direction.
A sludge blanket with flocks created by flocculating suspension is characterized by dynamic balance determining the size of flocks at the given spot. By catching particles of suspension and by agglomeration the single flocks grow, whereas large flocks are desintegrated to smaller ones under the influence of hydrodynamic forces. The fluidized layer for its part affects the liquid flow, thus establishing feedback.
The continuous interception of the suspension results in the increase of the total volume of flocks and, accordingly, the superfluous flocks should be removed from the sludge blanket. Thus the separated suspension is withdrawn from the sludge blanket in form of excess flocks.
Two types of sludge blankets are known: the fully fluidized one, specified also as perfectly fluidized, and the partially fluidized one, specified also as imperfectly fluidized. They differ as to the velocity of liquid at the surface of the sludge blanket and as to the type of withdrawal of excess flocks. In a partially fluidized sludge blanket the velocity of liquid at the surface of the sludge blanket is smaller than the limit of fluidization and the excess flocks are withdrawn from the bottom, in a fully fluidized sludge blanket the velocity of liquid at the surface of the sludge blanket exceeds the limit of fluidization and the excess flocks are withdrawn from the surface of the sludge blanket.
Due to the fact that the velocity of liquid tends to be slower than the fluidization limit at the surface of the partially fluidized sludge blanket, failures of fluidization are encountered there. Large agglomerations of flocks are created that fall down through the fluidized layer. Their falling down leads to rising currents in the vicinity, thus increasing the local velocity of upward flow, which contributes to the maintaining of fluidization in other zones close to the sludge blanket surface. Since the average velocity of the upward flow in a fluidized layer increases in the downward direction, some agglomerates are decomposed in the quicker flow and their flocks return back to the sludge blanket. Some agglomerates, however, fall through under the fluidized layer wherefrom they are removed. Within a certain range of parameters a balance is achieved between the amount of suspension flowing into the sludge blanket and the amount of suspension falling out of the sludge blanket and withdrawn by way of the described mechanism. If the amount of incoming suspension exceeds the amount of suspension that falls out, the volume of the sludge blanket increases, and if it exceeds the capacity of the plant, the sludge blanket starts being washed away into the withdrawal of purified water, i.e., it flows over. If the amount of incoming suspension is smaller than the amount of the suspension falling out, the volume of sludge blanket decreases, and if it drops under a critical value, the sludge blanket drops under the separator or, in other words, it falls out of the separation space.
The concentration of flocks in the sludge blanket depends upon the velocity of the upward flow. The lower is the flow velocity the higher is the concentration. The concentration of flocks in the agglomerates falling out of a partially fluidized sludge blanket is higher than what would correspond to the velocity of fluidization limit. That is why the concentration of separated suspension removed from a partially fluidized sludge blanket can be higher than the concentration of a suspension removed from a fully fluidized sludge blanket. On the other hand, however, the flow velocity at the surface of the sludge blanket and, accordingly, the hydraulic performance of a fully fluidized sludge blanket is higher than that of a partially fluidized sludge blanket. That is why the use of fully fluidized sludge blanket is favorable for the separation of diluted suspensions, whereas the partially fluidized sludge blanket is suitable for separating concentrated suspensions.
For this reason the fully fluidized sludge blanket has been used in the chemical treatment of water where the concentration of suspension, as a rule, makes tenths of grams of dry matter per cubic meter. The velocity of liquid flow at the surface of the sludge blanket achieves currently the values of 4-4.5 m per hour while the suspension withdrawn from the surface of the sludge blanket is four times to eight times thicker, the withdrawn flocks being later subjected to secondary thickening by sedimentation. A partially fluidized sludge blanket can be used in biological treatment of sewage where current concentrations of the suspension make 4 to 6 kg of dry matter per cubic meter and the separated thickened suspension is returned back into the treatment process. The flow velocity of liquid at the surface of the sludge blanket currently achieves values of 0.8-1 meter per hour and the withdrawn suspension may thicken from 1.5 times up to the double.
Of course all limit values depend upon a number of parameters, of which especially the water temperature and the character of suspension have remarkable influence. By monitoring many plants over a number of years these parameters were found to influence the limit values within 10 to 30 percent, as a rule.
The separation spaces wherein the described filtration in the sludge blanket takes place have usually the form of an upwards broadening cone, pyramid or prism, ensuring the decrease of liquid flow velocity in the upward direction. They are limited by inclined walls, usually 52° to 60° inclination which, on the one hand side, prevents flocks from depositing layers on these walls and, on the other hand, it provides sufficient surface for the surface of sludge blanket. The stream of liquid in these separation zones has, due to their shape, in addition to the vertical upward component, also a horizontal component directed to the inclined walls. Against the vertical component of flow the flocks are subjected to gravitation forces in downward direction. Being combined these forces result in a horizontal force that urges the flocks in direction to the inclined walls. Owing to that the concentration of suspension increases at the inclined walls, resulting in downward density streams along these walls. In a partially fluidized sludge blanket the agglomerates of flocks falling down continue, after having contacted the inclined wall, also as density streams. The concentration of suspension in the density streams is then further influenced by two contrary effects: on the one hand, due to the gravitation force, further thickening of the suspension takes place in the density stream flowing down along an inclined wall; on the other hand the counterflow of liquid streaming towards the separation space in the upward direction washes through the density flow diluting, on the contrary, the suspension in the density flow.
The separators for the sludge blanket are further equipped with the withdrawal of pure liquid without suspension at the top, usually in the form of overflow troughs or perforated tubes, and at the bottom they are provided with inlet of liquid with suspension to be separated.
The simplest solution of this inlet is a simple hole connecting the separation space with another functional space, such as an activation space in case of biological waste treatment or a coagulation space in case of chemical water treatment. However, also more complex solutions are known, such as in form of inclined feeding channels along the walls of the separation space, or in form of a central inlet pipe passing vertically through the center of the separation space. Such inlet channels or pipes are then connected with another functional space from which the liquid with suspension usually flows down to the spot of the actual entry to the separation space in which the liquid flows upwards. If the overall arrangement of the entry into the separation space is more complex, then, with regard to the above described mechanism of hydrodynamic support of the fluidized layer of the sludge blanket, under the concept of entry to the separation space the horizontal surface is understood at the upper level of the hole through which water flows to such inlet to the separation space. The upper part of the separation space for a fully fluidized sludge blanket is provided with withdrawal of separated suspension delimiting the position of the sludge blanket surface, whereas for a partially fluidized sludge blanket the withdrawal of separated suspension is arranged under the level of entry of the liquid with suspension to the separation space. The throughflow area of the liquid with suspension entry to the separation space, as a rule, makes 2.2 to 2.5 percent of the separation space for a fully fluidized sludge blanket, and 10 to 15 percent of the same for a partially fluidized sludge blanket. The larger the throughflow area of the entry to the separation space in a partially fluidized sludge blanket, the higher concentrations of suspension can be separated by this sludge blanket, but the higher also the limit for this sludge blanket to fall out.
The described principles elucidate yet another substantial difference between a partially fluidized sludge blanket and a fully fluidized one. The height of sludge blanket surface in a fully fluidized sludge blanket is constant, and if there are any changes of throughflow or concentration of the entering suspension, only the concentration of withdrawn thickened suspension varies. Exceeding the maximum performance is manifested by taking flocks out of the sludge blanket and by its surface being washed out. In a partially fluidized sludge blanket its surface height varies along with changes of throughflow and of concentration of the entering suspension, and exceeding the maximum performance is manifested by the rise of the sludge blanket up to the withdrawal level of purified liquid, with following overflow of the sludge blanket to the withdrawal.
Operation experience has shown the sludge blanket is properly functional always within a certain range of design parameters only. If the throughflow drops under about 50 percent of the rated performance in a fully fluidized sludge blanket used for chemical water treatment, disturbances of fluidization occur that have the tendency to get worse, and within a certain time they result in functional failures. If the concentration of activated sludge drops under 1-2 kg of dry matter per cubic meter in case of a partially fluidized sludge blanket used for biological treatment of water, a sludge blanket is not established in the separation space, or if the concentration of suspension has dropped under the mentioned limit, the sludge blanket is likely to fall out of the separation space, i.e., it will sink under the separation space.
The principles of fully fluidized sludge blanket and various arrangements of corresponding apparatuses are described, e. g., in the Czech Patent Specification No 88634 (S. Mackrle, V. Mackrle, I. Tesarik, V. Mican, Reactor for water treatment by sludge blanket) and the Czech Patent Specification No 123929 (S. Mackrle, V. Mackrle, O. Dracka, L. Paseka, Clarifier for water treatment by coagulation and filtration by perfectly fluidized sludge blanket) and its corresponding Canadian Patent Specification No 769769. A partially fluidized sludge blanket with spontaneous falling down of separated suspension back to the treatment process is described, e. g., in the Czech Patent Specification No 159811 (S. Mackrle, V. Mackrle Modular apparatus for biological treatment of organically polluted liquids) and its corresponding foreign patent specifications, the Canadian No 921626 and the U.S. Pat. No. 3,627,136, and is also described in the Czech Patent Specification No 173893 (S. Mackrle, V. Mackrle, O. Dracka, Reactor for biological purification of liquid, in particular sewage water) and its corresponding foreign patent specifications, the Canadian No 1038090, German No 2456953, French No 7439337 and the Japanese No 1044405. A partially fluidized sludge blanket with the application of sucking away the fallen down separated suspension is described in the Czech Patent Specification No 275746 (S. Mackrle, V. Mackrle, Method of biological activation purification of water and apparatus for performing the same), with corresponding U.S. Pat. No. 5,032,276 and EP 345669.
The drawbacks of the prior art are substantially eliminated by the method according to the present invention characterized in that the thickened separated suspension in form of flocks from the sludge blanket is withdrawn from the zone of the fluidized layer, the velocity of upward flow in the fluidized layer decreasing essentially in the upward direction.
It is beneficial if the thickened separated suspension in form of flocks of the sludge blanket is withdrawn from an outer boundary zone of the fluidized layer and if the velocity of flow in the upward direction decreases both above the level of the withdrawal of the thickened suspension and under the same.
It is further important that the layer of the sludge blanket above the withdrawal level of the thickened suspension functions as a partially fluidized sludge blanket wherein agglomerates of thickened suspension are established that are then removed, the layer of sludge blanket under the withdrawal level of thickened suspension functioning as a fully fluidized sludge blanket wherein the liquid flow is distributed into the partially fluidized sludge blanket.
It is preferable for reducing the volume of withdrawn excess suspension if the separated thickened suspension removed from the fluidized layer forcibly moves downward while getting further thickened, and, if the concentration of inflowing suspension exceeds 1 kg of dry matter per cubic meter, the velocity of upward water flow immediately above the surface of sludge blanket is in the range of 1.6 to 2.2 meters per hour and the water flow velocity at the entrance to the sludge blanket is within the range of 2 to 6 cm per second. The volume of withdrawn thickened suspension makes 1.5 multiple to 3 multiple of the volume of water without suspension withdrawn above the surface of the sludge blanket.
The object of the apparatus according to the invention for performing the described method consists in that the separator, the inner volume of which contains the separation space, is provided by at least one withdrawal spot of thickened suspension that is located above the inlet to the separator, predominantly at its outer wall or outer walls and under the surface of the sludge blanket.
It is also substantial that the withdrawal spot of the thickened suspension are vertically located in the middle part of the separation space, close to at least one of its outer walls, while the separation space within the separator essentially widens in the upward direction both above the level of withdrawal of the thickened suspension and underneath the same.
According to another variant of the apparatus according to the invention it is important that the separation space within the separator, in its bottom part, is limited at least by one, at least partially inclined inner wall, while the space between the bottom part of the outer wall and the inner wall creates a thickening space, whereas the gap between the upper edge of this inner wall and the outer wall represents the withdrawal spot of thickened suspension from the separation space.
Along with that it is beneficial if the gap between the upper edge of the inner wall and the outer wall also creates an entry to the thickening space that is provided with means for withdrawing the thickened suspension in its bottom part.
Yet another variant is preferable wherein the means for withdrawing thickened suspension are created by a horizontally arranged collecting tube arranged adjacent to the inclined outer wall of the separator.
A contribution is offered also by an embodiment wherein the inclined outer wall of the separator makes an angle in the withdrawal zone of thickened suspension, the upper part above this level being more inclined than the bottom part of the same underneath.
Considering the effectiveness of removing the thickened suspension, it is beneficial if the separator, and consequently also the separation space, suddenly widens upwards at the place of collecting tubes, while the side of collecting tubes turned to the upper part of shifted inclined outer wall is provided with apertures.
It is advantageous for the functioning of the apparatus according to the invention that the area of entrance to the separation space makes more than 3 percent and less than 6 percent of the surface of the separation space at the level of withdrawal of liquid without suspension, whereas the area of the separation space immediately under the removal level of thickened suspension makes more than 20 percent, and immediately above the level of thickened suspension removal it makes less than 70 percent of the surface of the separation space at the level of withdrawing liquid without suspension. It is also preferable to maintain a vertical distance of more than one meter between the withdrawal level of thickened suspension and both the height of entry to the separation space and the height of withdrawing the liquid without suspension.
It is also significant that the height of the withdrawal level of thickened suspension above the level of entry into the separation space is in the range from ¼ to ¾ of the height of withdrawing liquid without suspension above the entry level into the separation space.
Considering the design it is a contribution that at least one functional tube from the group created by the collecting tubes of the thickened suspension, collecting tubes for withdrawing the thickened suspension, the collecting tubes for withdrawing liquid without suspension, the tubes serving as discharge, the inlet pipes of pressure air and the rinsing pipes, creates also a part of the supporting structure of the outer walls of the separation space.
It is also advantageous if the angle of the upper part of the inclined outer wall is within the range between 52° and 60° or, possibly, if the angle of the inclined inner wall is within the range between 52° and 60°, whereas the angle of the bottom part of the inclined outer wall is within the range of 30° to 40°.
The most essential advantage of the method and the apparatus according to the present invention is a substantial improvement of the efficiency of separation, which is enabled in particular by the increase of solids load of the separation when separating a concentrated suspension, and namely up to the double achievable by known systems of fluid filtration using a partially fluidized sludge blanket. This can be made use of either for increasing the hydraulic load and, accordingly, for enhancing the separation capacity, or for increasing the concentration of suspension entering the sludge blanket or, possibly, for an optimum combination of both these effects. Such quantitative improvement of separation efficiency will be a special contribution for the activation type of biological waste water treatment regarding the savings in the design of integrated biological reactors. The increase of hydraulic load owing to the application of the method and apparatus according to the present invention allows to cut down the separation space, and namely by up to 50 percent against the dimensions of hitherto known plants using a partially fluidized sludge blanket. This brings not only savings relating to the construction of the separator, but also further construction savings, such as by reducing the necessary height of the integrated biological reactor and easier accommodation of the separator in the reactor. The increased concentration of activated sludge in the biological reactor is also reflected in cutting down the functional volumes that are necessary for biological processes and thereby also cutting down the overall size of the reactor. The reduction of the separator size and the optimization of the construction and of the dimensions of the reactor allow to achieve considerable savings of material, manufacturing cost, transport, and installation. Another advantage of the method and apparatus for implementing the method according to the present invention is their functioning within a substantially broader range of parameters than in the case of a partially fluidized sludge blanket. This widens the scope of utilization of the method and the apparatus and enables their substantially improved flexibility during operation.